www.afm-journal.de FULL PAPER © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 4592 www.MaterialsViews.com wileyonlinelibrary.com Changzhou Yuan, Jiaoyang Li, Linrui Hou, Xiaogang Zhang, Laifa Shen, and Xiong Wen (David) Lou* 1. Introduction In recent years, electrochemical capacitors (ECs), also called supercapacitors, have attracted tremendous interest as power sources for applications requiring quick bursts of energy, such as high power electronic devices and electric vehicles. ECs are able to deliver higher power density with better cycling lifespan over batteries, and store more energy than conventional capaci- tors. ECs commonly store energy based on either ion adsorp- tion (electrochemical double layer capacitors, EDLCs) or fast surface redox reactions (pseudocapacitors). [1,2] Unfortunately, the low specific capacitance (SC) of EDLCs cannot meet the ever-growing need for peak-power assist- ance in electric vehicles, and so on. Thus, growing interest in using pseudocapaci- tive materials for ECs has been triggered because the energy density associated with Faradaic reactions is substantially larger by at least one order of magnitude than that of EDLCs. [3–5] In common, pseudocapaci- tive materials, which mainly include metal hydroxides, oxides and conductive poly- mers, possess multiple oxidation states/ structures that are capable of rich redox reactions. One of the most notable pseu- docapacitive materials studied is RuO 2 . However, its large-scale application is hin- dered by the very high cost and rareness of the Ru element. [6,7] Among many metal oxides, spinel nickel cobaltite (NiCo 2 O 4 ) has been conceived as a promising cost- effective and scalable alternative since it offers many advantages such as low cost, abundant resources and environmental friendliness. [4,8–12] More importantly, it is reported that spinel NiCo 2 O 4 possesses much better electrical conductivity, at least two orders of magnitude higher, and higher electrochemical activity than nickel oxides or cobalt oxides. [13,14] It is therefore expected to offer richer redox reactions, including contributions from both nickel and cobalt ions, than those of the monometallic nickel oxides and cobalt oxide. [4,8–12] These attractive features are of great advantage for its application in high-performance ECs. To maximize the electrochemical performance of a pseudoca- pacitor, one needs to engineer the electrodes with large amount of electroactive sites and high transport rates for both electrolyte ions and electrons that simultaneously take part in the Faradaic reactions. [5] More specifically, the former requires large specific surface area (SSA) of electroactive materials, which will pro- mote the electric double-layer capacitance and accommodate a large amount of superficial electroactive species for participa- tion in the Faradaic redox reactions. While the later entails fast diffusion of the electrolyte ions and fast conduction of electrons to the electroactive sites. This can be achieved by concocting mesoporous porosity into the electroactive materials with large naked SSA, high electrical conductivity and fast ion transport. However, NiCo 2 O 4 -based electrodes are commonly binder- enriched electrodes made by the traditional slurry-coating technique for electrochemical evaluation, [4,8–12] where a large portion of the electroactive NiCo 2 O 4 surface is “dead surface” and blocked from the contact with the electrolyte to participate Ultrathin Mesoporous NiCo 2 O 4 Nanosheets Supported on Ni Foam as Advanced Electrodes for Supercapacitors A facile two-step method is developed for large-scale growth of ultrathin mesoporous nickel cobaltite (NiCo 2 O 4 ) nanosheets on conductive nickel foam with robust adhesion as a high-performance electrode for electrochemical capacitors. The synthesis involves the co-electrodeposition of a bimetallic (Ni, Co) hydroxide precursor on a Ni foam support and subsequent thermal trans- formation to spinel mesoporous NiCo 2 O 4 . The as-prepared ultrathin NiCo 2 O 4 nanosheets with the thickness of a few nanometers possess many interpar- ticle mesopores with a size range from 2 to 5 nm. The nickel foam supported ultrathin mesoporous NiCo 2 O 4 nanosheets promise fast electron and ion transport, large electroactive surface area, and excellent structural stability. As a result, superior pseudocapacitive performance is achieved with an ultrahigh specific capacitance of 1450 F g -1 , even at a very high current density of 20 A g -1 , and excellent cycling performance at high rates, suggesting its promising application as an efficient electrode for electrochemical capacitors. DOI: 10.1002/adfm.201200994 Dr. C. Z. Yuan, J. Y. Li, L. R. Hou Anhui Key Laboratory of Metal Materials and Processing School of materials Science and Engineering Anhui University of technology Ma `anshan, 243002, P. R. China Dr. C. Z. Yuan, Prof. X. W. Lou School of Chemical and Biomedical Engineering Nanyang Technological University 70 Nanyang Drive, Singapore 637457 E-mail: xwlou@ntu.edu.sg Prof. X. G. Zhang, L. F. Shen College of Material Science & Engineering Nanjing University of Aeronautics and Astronautics Nanjing, 210016, P. R. China Adv. Funct. Mater. 2012, 22, 4592–4597